Thyristor operated dimmer:
A popular thyristor operated dimmer circuit employ TRIAC-DIAC phase control for low AC power control. During each half-cycle, the capacitor charges through (R1+VR1). When the capacitor voltage becomes equal to a breakover voltage of diac, the diac conducts. The capacitor then discharges through diac and gate-MT1 junction of triac. The discharge current provides gate current to triac to turn it on. After turn-on of triac, it acts like short-circuit and supply voltage appears across lamp. For rest of the half-cycle, the supply voltage appears across the lamp. If the triac is fired early in the half-cycle, the supply voltage appears across lamp for longer period and lamp glows brighter. However, if the triac is fired later in the half-cycle, the supply voltage appears across lamp for lesser period and lamp glows dim. The firing instant is decided by the capacitor charging to breakover voltage of diac. i.e charging time constant {C (R1+VR1)}. Thus by reducing VR1 the lamp brightness can be increased.